1. Field of the Invention
The present invention concerns an electronic device for thermal protection of a load comprising current transformers installed on the polyphase line wires supplying power to said load and Connected to an electronic processor circuit generating a command to interrupt the polyphase power supply to Said load.
2. Discussion of the Background
Electric motors are protected by overload relays associated with contactors or thermal protection relays associated with circuit-breakers. These devices monitor the current drawn on each phase and sense absence of any phase or imbalance between phases. They usually include a bimetal strip for each phase heated by the phase current. If an overload occurs on a phase the bimetal strip for the overloaded phase bends and commands disconnection of the power supply to the motor. A differential device commands tripping in response to absence of a phase or to an imbalance between phases.
Overload relays are difficult to use for currents exceeding around 100 amperes. The heating of each bimetal strip and therefore the tripping current are not accurately controlled because of unwanted heating that occurs.
Electric motors are also protected by electronic relays such as that described in French patent 2 586 489. A relay of this kind includes a current sensor for each phase producing an image of the phase current. An electronic processor circuit receives and processes the phase current images and generates a tripping command in response to an overload or an imbalance between phases.
French patent 2 641 410 describes an electronic relay fitted with a heat detector including a resistive heater element through which a phase current flows and which is thermally coupled to a temperature sensor by means of a thermal coupling body. This heat detector provides a simple way of storing the thermal status of the load protected by the relay.